On 1/14/19 6:27 AM, Arnaldo Carvalho de Melo wrote:
> Em Sat, Jan 12, 2019 at 12:00:42AM +0000, Yonghong Song escreveu:
>> On 1/10/19 3:03 PM, Andrii Nakryiko wrote:
>>> There appears to be problems with bitfields in both DWARF data and
>>> pahole's conversion from DWARF to BTF (see test file source code and
>>> pahole output at the bottom).
>
>>> 1. pahole believes unpacked.y1 has offset 7, which is incorrect
>>> (should be 32?).
>>> unpacked.y2 has offset 32, which also doesn't make sense.
>>> DWARF info for unpacked.y1 and unpacked.y2 also seem wrong (how
>>> do we get offset 18 for y1?).
>
>>> <2><63>: Abbrev Number: 3 (DW_TAG_member)
>>> <64> DW_AT_name : y1
>>> <67> DW_AT_decl_file : 1
>>> <68> DW_AT_decl_line : 5
>>> <69> DW_AT_type : <0x87>
>>> <6d> DW_AT_byte_size : 4
>>> <6e> DW_AT_bit_size : 7
>>> <6f> DW_AT_bit_offset : 18
>>> <70> DW_AT_data_member_location: 0
>>> <2><71>: Abbrev Number: 3 (DW_TAG_member)
>>> <72> DW_AT_name : y2
>>> <75> DW_AT_decl_file : 1
>>> <76> DW_AT_decl_line : 6
>>> <77> DW_AT_type : <0x87>
>>> <7b> DW_AT_byte_size : 4
>>> <7c> DW_AT_bit_size : 20
>>> <7d> DW_AT_bit_offset : 12
>>> <7e> DW_AT_data_member_location: 4
>>>
>>>
>>> 2. Similar problems with packed.y1 and packed.y2. packed.y2 offset
>>> is negative value even in DWARF:
>>>
>>> <2><d2>: Abbrev Number: 6 (DW_TAG_member)
>>> <d3> DW_AT_name : y2
>>> <d6> DW_AT_decl_file : 1
>>> <d7> DW_AT_decl_line : 14
>>> <d8> DW_AT_type : <0x87>
>>> <dc> DW_AT_byte_size : 4
>>> <dd> DW_AT_bit_size : 20
>>> <de> DW_AT_bit_offset : -2
>>> <df> DW_AT_data_member_location: 0
>>
>> Did some research on why we got -2 as bit_offset with the above packed
>> data structure.
>>
>> First, the compiler generates correct thing. the negative
>> DW_AT_bit_offset is allowed for dwarf2, but not dwarf4.
>
>> The "-2" here means (1). the member is crossing two
>> adjacent "4" bytes boundary (DW_AT_byte_size = 4), and
>
> Ok, but I would expect that the DW_AT_data_member_location would be 4,
> meaning, it is mostly on this one but starts two bits before. So when
Unfortunately, this is not dwarf interpretation for little endian.
(I wished it be as easy as big endian...).
-bash-4.4$ cat t.c
struct t {
int a:3;
int b:29;
} g;
-bash-4.4$ gcc -O2 -g -c t.c
-bash-4.4$ llvm-dwarfdump t.o
...
0x00000027: DW_TAG_member
DW_AT_name ("a")
DW_AT_decl_file ("/home/yhs/tmp2/t.c")
DW_AT_decl_line (2)
DW_AT_type (0x00000042 "int")
DW_AT_byte_size (0x04)
DW_AT_bit_size (0x03)
DW_AT_bit_offset (0x1d)
DW_AT_data_member_location (0x00)
0x00000034: DW_TAG_member
DW_AT_name ("b")
DW_AT_decl_file ("/home/yhs/tmp2/t.c")
DW_AT_decl_line (3)
DW_AT_type (0x00000042 "int")
DW_AT_byte_size (0x04)
DW_AT_bit_size (0x1d)
DW_AT_bit_offset (0x00)
DW_AT_data_member_location (0x00)
The DW_AT_data_member_location is 0 for both members.
The first member offset 0x1d is from the byte_size boundary (4 bytes).
The second member offset 0x0 is also from the byte_size boundary (4 bytes).
For the above packed case, it merely mean that
the offset -2 from the byte_size_boundary (4 bytes), but instead go into
the byte, rather going opposite directions with 2 bits.
An alternative way to represent with data_member_location 4, but
bit_offset will have to change.
old way:
>>> <dc> DW_AT_byte_size : 4
>>> <dd> DW_AT_bit_size : 20
>>> <de> DW_AT_bit_offset : -2
>>> <df> DW_AT_data_member_location: 0
new way:
<dc> DW_AT_byte_size : 4
<dd> DW_AT_bit_size : 20
<de> DW_AT_bit_offset : 30
<df> DW_AT_data_member_location: 4
The compiler did not generate the new way.
> seeing negative DW_AT_bit_offsets we should think it starts in this
> location (0 in this case), but straddles to the next, so I should say
> that it is at bit offset (DW_AT_byte_size * 8 - DW_AT_bit_offset), the
> user, looking at this new pahole output will realize that it has 2 bits
> at the end of the current DW_AT_data_member_location (0) but has 18 bits
> in the next.
>
>
>> (2). the -2 means the the starting bit is at the "4" byte boundary - 2
>> bits. So basically it covers:
>>
>> | |
>> V V
>> [0 1 .....30 31][0 .. 17 18 ... 31]
>>
>> totally 20 bits.
>>
>> The following patch fixed the issue:
>> -bash-4.4$ git diff
>> diff --git a/dwarves.h b/dwarves.h
>> index e6bffe8..e5f8347 100644
>> --- a/dwarves.h
>> +++ b/dwarves.h
>> @@ -831,7 +831,7 @@ struct class_member {
>> uint32_t bit_size;
>> uint32_t byte_offset;
>> size_t byte_size;
>> - uint8_t bitfield_offset;
>> + int8_t bitfield_offset;
>> uint8_t bitfield_size;
>> uint8_t bit_hole;
>> uint8_t bitfield_end:1;
>> -bash-4.4$
>>
>> With the above change:
>> -bash-4.4$ ~/work/github/pahole/build/pahole -JV bitfields.o
>> File bitfields.o:
>> [1] INT int size=4 bit_offset=0 nr_bits=32 encoding=SIGNED
>> [2] STRUCT packed kind_flag=1 size=5 vlen=5
>> x1 type_id=3 bitfield_size=1 bits_offset=0
>> x2 type_id=3 bitfield_size=3 bits_offset=1
>> x3 type_id=3 bitfield_size=3 bits_offset=4
>> y1 type_id=1 bitfield_size=7 bits_offset=7
>> y2 type_id=1 bitfield_size=20 bits_offset=14
>> [3] INT char size=1 bit_offset=0 nr_bits=8 encoding=(none)
>> [4] STRUCT unpacked kind_flag=1 size=8 vlen=5
>> x1 type_id=3 bitfield_size=1 bits_offset=0
>> x2 type_id=3 bitfield_size=3 bits_offset=1
>> x3 type_id=3 bitfield_size=3 bits_offset=4
>> y1 type_id=1 bitfield_size=7 bits_offset=7
>> y2 type_id=1 bitfield_size=20 bits_offset=32
>> [5] UNION packed_union kind_flag=0 size=8 vlen=4
>> x1 type_id=3 bits_offset=0
>> y2 type_id=1 bits_offset=0
>> s1 type_id=2 bits_offset=0
>> s2 type_id=4 bits_offset=0
>> [6] UNION unpacked_union kind_flag=0 size=8 vlen=3
>> x1 type_id=3 bits_offset=0
>> y2 type_id=1 bits_offset=0
>> s type_id=4 bits_offset=0
>> -bash-4.4$
>>
>> Will send out a patch shortly.
>>
>>>
>>> andriin@devvm$ cat bitfields.c
>>> struct unpacked{
>>> char x1: 1;
>>> char x2: 3;
>>> char x3: 3;
>>> int y1: 7;
>>> int y2: 20;
>>> };
>>>
>>> struct packed{
>>> char x1: 1;
>>> char x2: 3;
>>> char x3: 3;
>>> int y1: 7;
>>> int y2: 20;
>>> } __attribute__((packed));
>>>
>>> union packed_union {
>>> char x1;
>>> int y2;
>>> struct packed s1;
>>> struct unpacked s2;
>>> };
>>>
>>> union unpacked_union {
>>> char x1;
>>> int y2;
>>> struct unpacked s;
>>> } __attribute__((packed));
>>>
>>> int main() {
>>> struct packed s1;
>>> struct unpacked s2;
>>> union packed_union u1;
>>> union unpacked_union u2;
>>> return 0;
>>> }
>>> andriin@devvm$ cc -g bitfields.c -o bitfields
>>> andriin@devvm$ LLVM_OBJCOPY=objcopy ~/local/pahole/build/pahole -JV bitfields
>>> File bitfields:
>>> [1] STRUCT unpacked kind_flag=1 size=8 vlen=5
>>> x1 type_id=2 bitfield_size=1 bits_offset=0
>>> x2 type_id=2 bitfield_size=3 bits_offset=1
>>> x3 type_id=2 bitfield_size=3 bits_offset=4
>>> y1 type_id=3 bitfield_size=7 bits_offset=7
>>> y2 type_id=3 bitfield_size=20 bits_offset=32
>>> [2] INT char size=1 bit_offset=0 nr_bits=8 encoding=(none)
>>> [3] INT int size=4 bit_offset=0 nr_bits=32 encoding=SIGNED
>>> [4] STRUCT packed kind_flag=1 size=5 vlen=5
>>> x1 type_id=2 bitfield_size=1 bits_offset=0
>>> x2 type_id=2 bitfield_size=3 bits_offset=1
>>> x3 type_id=2 bitfield_size=3 bits_offset=4
>>> y1 type_id=3 bitfield_size=7 bits_offset=7
>>> y2 type_id=3 bitfield_size=255 bits_offset=16776974
>>> [5] UNION packed_union kind_flag=0 size=8 vlen=4
>>> x1 type_id=2 bits_offset=0
>>> y2 type_id=3 bits_offset=0
>>> s1 type_id=4 bits_offset=0
>>> s2 type_id=1 bits_offset=0
>>> [6] UNION unpacked_union kind_flag=0 size=8 vlen=3
>>> x1 type_id=2 bits_offset=0
>>> y2 type_id=3 bits_offset=0
>>> s type_id=1 bits_offset=0
>>> andriin@devvm$ readelf -wi bitfields
>>> Contents of the .debug_info section:
>>>
>>> ...
>>>
>>> <1><2d>: Abbrev Number: 2 (DW_TAG_structure_type)
>>> <2e> DW_AT_name : (indirect string, offset: 0x4e): unpacked
>>> <32> DW_AT_byte_size : 8
>>> <33> DW_AT_decl_file : 1
>>> <34> DW_AT_decl_line : 1
>>> <35> DW_AT_sibling : <0x80>
>>> <2><39>: Abbrev Number: 3 (DW_TAG_member)
>>> <3a> DW_AT_name : x1
>>> <3d> DW_AT_decl_file : 1
>>> <3e> DW_AT_decl_line : 2
>>> <3f> DW_AT_type : <0x80>
>>> <43> DW_AT_byte_size : 1
>>> <44> DW_AT_bit_size : 1
>>> <45> DW_AT_bit_offset : 7
>>> <46> DW_AT_data_member_location: 0
>>> <2><47>: Abbrev Number: 3 (DW_TAG_member)
>>> <48> DW_AT_name : x2
>>> <4b> DW_AT_decl_file : 1
>>> <4c> DW_AT_decl_line : 3
>>> <4d> DW_AT_type : <0x80>
>>> <51> DW_AT_byte_size : 1
>>> <52> DW_AT_bit_size : 3
>>> <53> DW_AT_bit_offset : 4
>>> <54> DW_AT_data_member_location: 0
>>> <2><55>: Abbrev Number: 3 (DW_TAG_member)
>>> <56> DW_AT_name : x3
>>> <59> DW_AT_decl_file : 1
>>> <5a> DW_AT_decl_line : 4
>>> <5b> DW_AT_type : <0x80>
>>> <5f> DW_AT_byte_size : 1
>>> <60> DW_AT_bit_size : 3
>>> <61> DW_AT_bit_offset : 1
>>> <62> DW_AT_data_member_location: 0
>>> <2><63>: Abbrev Number: 3 (DW_TAG_member)
>>> <64> DW_AT_name : y1
>>> <67> DW_AT_decl_file : 1
>>> <68> DW_AT_decl_line : 5
>>> <69> DW_AT_type : <0x87>
>>> <6d> DW_AT_byte_size : 4
>>> <6e> DW_AT_bit_size : 7
>>> <6f> DW_AT_bit_offset : 18
>>> <70> DW_AT_data_member_location: 0
>>> <2><71>: Abbrev Number: 3 (DW_TAG_member)
>>> <72> DW_AT_name : y2
>>> <75> DW_AT_decl_file : 1
>>> <76> DW_AT_decl_line : 6
>>> <77> DW_AT_type : <0x87>
>>> <7b> DW_AT_byte_size : 4
>>> <7c> DW_AT_bit_size : 20
>>> <7d> DW_AT_bit_offset : 12
>>> <7e> DW_AT_data_member_location: 4
>>> <2><7f>: Abbrev Number: 0
>>> <1><80>: Abbrev Number: 4 (DW_TAG_base_type)
>>> <81> DW_AT_byte_size : 1
>>> <82> DW_AT_encoding : 6 (signed char)
>>> <83> DW_AT_name : (indirect string, offset: 0xb3): char
>>> <1><87>: Abbrev Number: 5 (DW_TAG_base_type)
>>> <88> DW_AT_byte_size : 4
>>> <89> DW_AT_encoding : 5 (signed)
>>> <8a> DW_AT_name : int
>>> <1><8e>: Abbrev Number: 2 (DW_TAG_structure_type)
>>> <8f> DW_AT_name : (indirect string, offset: 0x50): packed
>>> <93> DW_AT_byte_size : 5
>>> <94> DW_AT_decl_file : 1
>>> <95> DW_AT_decl_line : 9
>>> <96> DW_AT_sibling : <0xe1>
>>> <2><9a>: Abbrev Number: 3 (DW_TAG_member)
>>> <9b> DW_AT_name : x1
>>> <9e> DW_AT_decl_file : 1
>>> <9f> DW_AT_decl_line : 10
>>> <a0> DW_AT_type : <0x80>
>>> <a4> DW_AT_byte_size : 1
>>> <a5> DW_AT_bit_size : 1
>>> <a6> DW_AT_bit_offset : 7
>>> <a7> DW_AT_data_member_location: 0
>>> <2><a8>: Abbrev Number: 3 (DW_TAG_member)
>>> <a9> DW_AT_name : x2
>>> <ac> DW_AT_decl_file : 1
>>> <ad> DW_AT_decl_line : 11
>>> <ae> DW_AT_type : <0x80>
>>> <b2> DW_AT_byte_size : 1
>>> <b3> DW_AT_bit_size : 3
>>> <b4> DW_AT_bit_offset : 4
>>> <b5> DW_AT_data_member_location: 0
>>> <2><b6>: Abbrev Number: 3 (DW_TAG_member)
>>> <b7> DW_AT_name : x3
>>> <ba> DW_AT_decl_file : 1
>>> <bb> DW_AT_decl_line : 12
>>> <bc> DW_AT_type : <0x80>
>>> <c0> DW_AT_byte_size : 1
>>> <c1> DW_AT_bit_size : 3
>>> <c2> DW_AT_bit_offset : 1
>>> <c3> DW_AT_data_member_location: 0
>>> <2><c4>: Abbrev Number: 3 (DW_TAG_member)
>>> <c5> DW_AT_name : y1
>>> <c8> DW_AT_decl_file : 1
>>> <c9> DW_AT_decl_line : 13
>>> <ca> DW_AT_type : <0x87>
>>> <ce> DW_AT_byte_size : 4
>>> <cf> DW_AT_bit_size : 7
>>> <d0> DW_AT_bit_offset : 18
>>> <d1> DW_AT_data_member_location: 0
>>> <2><d2>: Abbrev Number: 6 (DW_TAG_member)
>>> <d3> DW_AT_name : y2
>>> <d6> DW_AT_decl_file : 1
>>> <d7> DW_AT_decl_line : 14
>>> <d8> DW_AT_type : <0x87>
>>> <dc> DW_AT_byte_size : 4
>>> <dd> DW_AT_bit_size : 20
>>> <de> DW_AT_bit_offset : -2
>>> <df> DW_AT_data_member_location: 0
>>> <2><e0>: Abbrev Number: 0
>>>
>>> ...
>>>
>
